Dual fuel engines are fuelled with two fuels simultaneously. Typically, a dual fuel engine is converted from a diesel engine and can operate in a diesel mode where the engine is fuelled with diesel and in a dual fuel mode where the engine is fuelled with a gaseous fuel and diesel simultaneously. A gaseous fuel is defined herein to be any fuel that is in the gas state at standard temperature and pressure, which in the context of this application is defined to be 20 degrees Celsius (° C.) and 1 atmosphere (atm). Exemplary gaseous fuels include biogas, butane, ethane, hydrogen, landfill gas, methane, natural gas and propane. In the dual fuel mode, the gaseous fuel is the primary fuel from which the engine derives the majority of its power and the diesel functions as a pilot fuel that is compression ignited such that the combustion of the pilot fuel ignites the gaseous fuel.
One distinction between conventional diesel engines and dual fuel engines is that when operating in a dual fuel mode the gaseous fuel is introduced into the intake air system, or intake ports or into the combustion chamber during the intake stroke or early in the compression stroke at relatively low pressure, compared to the pressure of the diesel fuel which is injected into the combustion chamber late in the compression stroke. The gaseous fuel forms a premixed air-fuel charge in the combustion chamber that when ignited burns with a premixed flame. Dual fuel engines often face challenges in controlling abnormal combustion events due to the inherent mismatch between alternative combustion modes of dual fuel operation and conventional diesel engine parameters, such as compression ratio, air flow rate, and combustion chamber geometry, that are optimized for diesel only operation. Three types of abnormal combustion are often encountered in a dual fuel engine: knock, pre-ignition and misfire.
Knock occurs when a large amount of unburned fuel-air mixture ignites spontaneously before the arrival of the propagating flame front from the normal ignition process. Knock can lead to a rapid rise of cylinder pressure and temperature, and strong pressure oscillation can occur within the combustion chamber. If not controlled promptly, knock can damage pistons and/or the cylinder head within a short period of time. Knock is particularly an issue for dual fuel engines because such engines often keep high compression ratios for maintaining the high thermal efficiency for base diesel operation.
Pre-ignition occurs when unburned air-fuel mixture in the combustion chamber is ignited by a heat source other than the normal ignition source (a spark plug or combustion of a pilot fuel), and happens before the normal ignition event. Hot spots such as those caused by carbon deposits on the surface of a combustion chamber are often found responsible for undesired early ignition. Since ignition by hot spots is not controlled by the normal ignition system, it often leads to erratic cylinder pressure change and higher peak cylinder pressure than that under normal conditions.
In comparison to conventional diesel engines, dual fuel engines that have premixed combustion modes are inherently more susceptible to knock and pre-ignition since the fuel is introduced and mixed well before the ignition event thereby increasing the likelihood of knock and pre-ignition, especially for fuels with lower octane numbers. In conventional diesel engines, fuel injection timing is related to ignition timing by a factor called ignition delay, which is the time required for compression ignition to occur in the combustion chamber after the diesel has been injected late in the compression stroke, and this value is typically a small number of crank angle degrees. Compared to premixed engines, conventional diesel engines have significantly less time to mix and to experience knock and pre-ignition.
Misfire can be caused by failed pilot ignition or issues with the air-fuel ratio of the mixture in the combustion chamber, such as the mixture being too lean or too rich. Misfire is a possible sign of fuel system failure, which should be dealt with promptly before causing more severe damage to the engine. The challenge of controlling misfire in a dual fuel engine is different from that for a conventional diesel engine because of the smaller quantities of pilot fuel, compared to when the engine is fuelled only with diesel fuel.
U.S. Pat. No. 4,478,068, issued to Bonitz et al. on Oct. 23, 1984 discloses an internal combustion engine knock sensing method and system that integrates a knocking signal over a measuring window, and then compares the integrated knocking signal with a reference signal to determine whether knocking is occurring. The reference signal is a weighted average of past values of the integrated knocking signal and the current integrated knocking signal. In this technique, knocking is determined to be occurring based on the history of combustion, and cannot be determined during a single combustion cycle solely based on information derived from the single combustion cycle. Furthermore, knocking can only be determined if there is a significant difference between knocking intensity from the current engine cycle and past engine cycles, otherwise minor increases in knocking intensity from cycle to cycle could cause a false positive indication. Severe engine knocking may not be determined when the knocking intensity ramps up slowly from engine cycle to cycle.
The state of the art is lacking in techniques for detecting and mitigating abnormal combustion characteristics. The present method and apparatus provide a technique for detecting abnormal combustion characteristics and performing a mitigation strategy associated with the detected abnormal combustion characteristic in internal combustion engines.